The motion of the crankshaft of a piston engine is not a purely smooth rotation. Superimposed on the rotation is an oscillatory or back and forth vibratory motion. This motion is caused by the power stroke pulses deflecting the crank pin in relation to either end of the crankshaft. As a cylinder fires, the power stroke of the related piston exerts force on a crankshaft in a direction that opposes force exerted by another cylinder that is in its compression stroke. The resulting torsion has numerous negative side effects on the engine. First and foremost is the fact that if undamped, these impulses and vibrations lead to higher crankshaft stresses which can cause the crankshaft to reach its fatigue limit after an unacceptably short service time. Secondly, if the valve train is driven by the crankshaft, as is the case in virtually all four-cycle engines, the cam timing and consequently valve event timing is altered. This oscillatory nature of the motion also brings about a random fluctuation of valve event timing and subsequent loss of power, and adversely affects the design dynamics of the cam profile. The cam profiles for a high rpm engine are designed to generate the minimum vibrations and impulses possible. Profile computations are based on the assumption that the crankshaft has smooth and steady rotational characteristics. When such vibrations and impulses occur the rpm of the valve train is adversely affected and the engine speed at the point where loss of controlled valve motion occurs is reduced. A third effect, of concern where the perceived smoothness of the engine is important, as is the case for most automobiles, is that unwanted crankshaft vibrations and impulses are felt by passengers and are perceived as rough running of the engine.
The usual technique to damp these unwanted vibrations and impulses is to mount a damper on one of the two ends of the crankshaft. Conventional dampers fall into three broad categories: the elastomeric damper; the viscous damper; and the pendulum damper. The elastomeric damper uses a mass, normally in the form of a ring bonded, by an elastomeric medium of rubber-like form, to a hub firmly attached to the front or rear of the crankshaft. As vibrations and impulses occur the elastomeric medium is put into shear first in one direction and then in the other. The internal friction of the elastomeric medium being high absorbs much of the energy and converts it to heat. The viscous damper works much in the same way as the elastomeric damper, but instead of using the elastomeric damping medium it uses the viscous shear of a mass immersed in a high viscosity fluid.
The pendulum damper incorporates a number of small masses of varying sizes housed within the damper or damping mechanism such that vibrational energy is transmitted into and out of these masses antiphase with the crankshaft vibration and impulse. As the rotation of the crankshaft accelerates, energy is absorbed by the pendulum mass and as the rotation of the crankshaft decelerates energy is given up and put back into the crankshaft.
Conventional damping techniques involve attaching a single damper to one of the ends of the crankshaft, achieving a limited damping effect. Vibrations, impulses and other forces must propagate the entire length of the crankshaft before being damped. A damper placed closer to the source of applied force would reduce the amount of vibration and torsional stress imposed on the crankshaft. Thus, conventional methods of attaching a single damper to one end of the crankshaft eliminates only a portion of the vibrations, impulses and other forces.
In addition to vibration and impulse damping, engine internals must also be balanced for smooth operation. Balancing the crankshaft to reduce main bearing stresses requires placing counter balance weights opposite each rod journal to offset the mass of the rod journal pin, the rod, bearings, wrist pin and piston/ring assembly. To balance the weight of the piston, rod, and journal with a counter weight, it must be considered that only mass directly opposite the rod journal delivers a full counter balancing effect. Because of the greater effectiveness of mass located in this area the total mass needed to balance the piston, rod and journal is minimized. Counter balance weights with increased density can concentrate more mass opposite the rod and journal, thus allowing less total counter balance weight to be used. Crankshafts often utilize metal inserts having greater density than the counter balance weight placed into bores incorporated in the counter balance weight to concentrate mass in the most effective area.
While damping is necessary for a smooth running crankshaft, all methods described thus far seek to damp out unwanted vibrations and impulses with a damper that is solely dedicated to the damping function. Conventional dampers add significant mass to an engine and, combined with heavy counter balance weights integrated into the crankshaft, inhibit acceleration and diminish engine performance. Also, the distance between the point where force is applied to the crankshaft and the counter balance weight increases the stress on the crankshaft.
There is a pressing need for a crankshaft damping mechanism which more effectively dampens unwanted vibration, impulse and other forces along the crankshaft while adding minimal mass to the engine.